It has been suggested that ethene can be dimerized to butenes in a reaction medium consisting of highly concentrated strong mineral acids such as phosphoric acid (concentration 14.0 mol/l, .about.85%) and sulfuric acid (concentration 17.0 mol/l, .about.95%) using as catalyst a palladium salt with an anion which has little or no interaction with the palladium. This dimerization is not attractive, however, because it combines a highly corrosive reaction medium with a very low conversion rate of ethene to butenes relative to the amount of palladium (cf.: USSR Patent Specification No. 374930 [Chem. Abstr. 90 22286 h]).
Further, it is known that ethene can be dimerized in chloroform using as catalyst the compound [Pd(CH.sub.3 CN)(PPh.sub.3).sub.3 ] (BF.sub.4).sub.2 or the compounds [Pd(CH.sub.3 CN).sub.4 ] (BF.sub.4).sub.2.n PPh.sub.3 (n=2,3) prepared in situ (cf.: J. Am Chem. Soc. 103 4627-4629 (1981) and 104 3520-3522 (1982)). These catalysts, however, require strictly anhydrous reaction conditions (cf.: Transition metal catalyzed polymerizations; Alkenes and dienes, Part A, Edited by R. P. Quirk, 1983 pp. 341-354).